\[(a)\;3s\quad (b)\;\frac{3}{2}s \quad (c)\;2s \quad (d)1 s\]

For each ball

$v^2-u^2=2as$

$\quad -u^2=2(-10)20$

$\qquad u=20 m/s$

also $v=u+at$

$\qquad 0=20-10 t$

$\qquad t=2 sec$

The ball has to return to his hand

so total time is 4 seconds

so each ball must be thrown at interval of $\large\frac{4}{4}$$=1 sec$ to maintain proper distance

Hence d is the correct answer.

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